One manner in which information is transmitted by radio waves is by amplitude modulating a radio-frequency carrier. In amplitude modulation (AM) the intensity or envelope of the radio-frequency carrier varies or is modulated by an information signal. At a receiver, the inverse process of demodulating or removing the information signal from the carrier signal, termed detection, is performed. The device which performs this demodulation in the receiver is termed a detector.
An amplitude modulation receiver includes an antenna which receives radio-frequency electromagnetic waves and converts them into radio-frequency electrical signals. These radio-frequency electrical signals are passed to a filter stage which prevents all signals, except those whose frequencies which are of interest, from passing through the filter stage onto the detector stage. The detector stage removes the carrier frequency signal while retaining the information encoded in the modulated portion of the signal.
Since the radio-frequency spectrum is broad and since the filter stage and the detector stage are most easily constructed for, and most stable over, a narrow range of frequencies, the receiver just described is generally not used. Instead, the typical receiver heterodynes, or multiplies the received radio-frequency signal by a second signal, termed a local oscillator or LO signal, of a predetermined frequency, to produce an intermediate frequency or IF signal having a much lower frequency than the radio-frequency or rf signal. This IF signal is then detected or demodulated to produce an audio-frequency signal.
In this manner, by varying the frequency of the local oscillator signal multiplying the received rf signal, the frequency of the intermediate frequency signal produced can remain constant over a wide range of rf frequencies. Therefore, the filter stage and the detector stage can be optimized for a narrow band of intermediate frequencies.
In such a heterodyne or superheterodyne receiver, in addition to the above mentioned components, there is also included a local oscillator and a multiplier. The local oscillator is typically tunable, unless only one rf frequency is to be detected, and produces the predetermined local oscillator or LO frequency signal. The multiplier combines or heterodynes this predetermined local oscillator frequency signal with the radio-frequency signal to produce two intermediate frequency signals whose frequencies are the sum and difference of the radio-frequency signal frequency and the local oscillator signal frequency.
Typically, it is the difference intermediate frequency that is desired and the difference frequency is separated from the sum frequency by passing the signals from the multiplier through a filter, similar to what is done in the simple AM receiver. The filtered intermediate frequency signal is then passed to the detector for demodulation. Such envelope detection is not only suitable for AM demodulation, but is also useful in encoded data demodulation and for carrier detection in a swept receiver.
A problem arises that at high frequencies and narrow band widths, it is difficult to build stable filters and detectors. The present invention relates to a very stable detector for use in envelope detection.